As the elderly population increases, senile dementia of the Alzheimer type (SDAT) is becoming one of the most devastating health problems facing society. Early in SDAT, individuals primarily produce memory deficits, however, these deficits are typically followed by widespread breakdowns in cognitive performance. The widespread involvement of cognitive processes in Alzheimer's disease suggests that there may be fundamental cognitive mechanisms involved in the progression of the disease. The goal of the present proposal is to continue our pursuit for such a fundamental mechanism. Specifically, the proposed studies all address the notion that SDAT individuals have difficulty inhibiting irrelevant information at different stages within the information processing system. Such a failure to inhibit the processing the irrelevant information has recently gained considerable attention in research addressing infants, children, and young and older adults. Recent evidence indicates that a breakdown in one's ability to inhibit irrelevant information can influence processes involved in perception, memory, comprehension, and even syntactic parsing. Furthermore, recent work completed in our lab (supported by Grant R01 AG03991) indicates that SDAT individuals fail to inhibit unbiased meanings of ambiguous words, and also fail to inhibit noncued locations in visual attentional tasks. In addition to providing information regarding failure to inhibit irrelevant information in SDAT individuals, the present work will also provide further information regarding the notion that healthy aged individuals produce some breakdown in their ability to inhibit irrelevant information. Each experiment will involve four groups of subjects (healthy young adults, healthy older adults, questionably demented individuals, and mildly demented individuals). The first experiments will address the phenomenon referred to as inhibition of return in a visual spatial processing task. The second experiment will address the negative impact of earlier ignored irrelevant information when it subsequently becomes relevant. Experiments 3 to 7 address processes involved in selecting meaning for ambiguous words in simple naming, word association, and in a relatedness judgement task. Each experiment will also include manipulations of stimulus onset asynchrony to track the time-course of inhibitory operations across the four groups of subjects. All experiments involve either simple key presses or vocal responses, and the resulting response latencies and error rates will be used to index underlying cognitive operations.